This invention pertains to still cameras, and more particularly pertains to an electronic control system for operating the shutter mechanism of such cameras in dependence upon ambient scene light.
Conventionally, still cameras are equipped with one or more blades which can be interposed between the camera objective and the film plane. When such blade or blades are completely closed, no light is transmitted to the film and therefore no exposure occurs. However, when the shutter is depressed and the blades are mechanically displaced, a beam of light of increasing width is directed upon the film as the blades move. Thus, initially, only a small amount of light reaches the film at the beginning of shutter opening, but this amount increases rapidly as the blade opens, until finally the blade or blades are fully opened. At this latter time, intensity of light incident upon the film reaches a maximum value.
It may thus be seen that proper film exposure is dependent upon two stages of shutter opening: a first stage, in which the shutter increases the amount of light which is incident upon the film; and a second stage in which the shutter is fully open. During the first stage, both the effective f-stop of the optical system increases along with exposure time, while during the second stage only exposure time varies since the maximum f-stop of the system has already been reached and cannot be exceeded. Moreover, it will be appreciated by those skilled in the art that it is the cumulative amount of light impinging upon the film which determines proper exposure.
During the second stage of shutter operation, the total amount of incident light directed upon the film will increase directly with time. Thus, light intensity need only be measured in a conventional fashion while the shutter is in the second stage of operation. However, during the first stage of operation, the amount of light incident upon the film will change with time, since during the first stage of shutter operation the amount of light incident upon the film increases from zero to whatever maximum amount of light the camera objective will admit. Thus, in this stage of operation it is necessary to weight the significance of light admitted during shutter opening since the total amount of light incident upon the film will be the sum of the amount of light incident upon the film during the first stage of shutter operation and the amount of light incident upon the film during the second stage of shutter operation.
It has been found that when a weighting factor of one-half is used, an accurate exposure will result. Thus, it would be advantageous to provide a device which would automatically regulate shutter operation by weighting light incident upon the film at a value of one-half while the shutter is in its first stage of operation and then to weight light incident upon the film during the second stage of shutter operation at a value of unity.